This invention relates to a method for manufacturing a mesa type semiconductor device used under pressure applied from the cathode side, i.e., in the pressure-applied manner.
Heretofore, there have been widely known and used pressure-applied type semiconductor devices which are used under pressure applied from the cathode side. As an example of such pressure-applied type semiconductor devices, FIG. 1 shows a pressure-applied type gate turn off thyristor device (hereinafter referred to as GTO). In FIG. 1, the GTO element is formed of a semiconductor body including an anode P layer 12, an intermediate N layer 14, a gate P layer 16, and a cathode N layer 18. The cathode layer 18 is divided into a plurality of mesa type portions 18a, 18b, 18c and 18d. The anode layer 12, gate layer 16, and cathode layer portions 18a, 18b, 18c and 18d are provided with an anode electrode 20, a gate electrode 22, and cathode electrodes 24a, 24b, 24c and 24d respectively. To the gate electrode 22 is attached a gate lead wire 25. Thus composed structure is supported by a supporting plate 26. An anode terminal plate 30 is attached to the supporting plate 26 by means of a solder 28. On the cathode electrodes 24a, 24b, 24c and 24d is disposed a pressing plate 32. The lateral width of the pressing plate 32 is smaller than a lateral width covering all the cathode electrodes 24a, 24b, 24c and 24d, that is, covering from the outer edge of one outermost cathode electrode 24a to the outer edge of the other outermost cathode electrode 24d. On the pressing plate 32 is disposed a cathode terminal plate 36. The lateral width of the pressing plate 32 is so prescribed as stated above for the following reason. That is, while the pressure-applied type semiconductor device is used under an external pressure applied from the cathode side, the cathode layer 18 subsides in such a manner as described later. Consequently, if the lateral width of the pressing plate 32 is large enough substantially to protrude outward from the cathode layer 18, the pressing plate 32 may come into contact with the gate lead wire 25 extending from the gate electrode 22, thereby causing short-circuit between the cathode and gate. The gate electrode 22 and cathode electrodes 24a to 24d are usually some 10 .mu.mm thick and the distance between the surface of the gate layer 16 and the surface of the cathode layer 18 is approximately 20 .mu.m, so that the distance between the under surface of the pressing plate 32 and the surface of the gate electrode 22 is approximately 20 .mu.m.
The GTO device (semiconductor device) may be used under a pressure F applied from the outside in such a direction as indicated by arrows in FIG. 2. That is, the device is used in such a manner that the thermal resistance between the cathode electrodes 24a, 24b, 24c and 24d and the pressing plate 32 is reduced, and the heat produced at the GTO element is positively radiated through the pressing plate 32 to increase the current capacity.
Meanwhile, when the external pressure F is applied through the pressing plate 32 as indicated by arrows, the soft solder 28 at a portion corresponding to the pressing plate 32 is plastically transformed as shown in FIG. 2. Namely, the central portion becomes thinner as compared with the peripheral portion. While in this case a portion of the GTO element corresponding to the pressing plate 32 is also transformed or sunk downward by the external pressure F, the portions of the outermost cathode electrodes 24a and 24d in contact with the pressing plate 32 are subjected to the external pressure F. Those portions which is not in contact with the pressing plate 32, however, is not subjected to the external pressure because the cathode electrodes 24a and 24d are only partially in contact with the pressing plate 32.
Thus, the outermost cathode layer portions 18a and 18d are so deformed as shown in FIG. 2, thereby concentrating the external pressure F upon the outermost cathode electrodes 24a and 24d.
If the cathode layer portions 18a and 18d are thus deformed, then there will be caused the following defects. That is,
(1) the effective area of the cathode electrodes 24a and 24d, i.e., the area of the cathode electrodes 24a and 24d which is in contact with the pressing plate 32 may be reduced, and the portions of the cathode electrodes 24a and 24d at the contact points may be raised in current density, thereby causing breakdown of the GTO element, and
(2) the metal (usually aluminium) forming the cathode electrodes 24a and 24d may be forced out under the influence of the heat produced in use to hang down along the side walls of the cathode layer portions 18a and 18d, thereby causing short-circuit between the cathode electrodes 24a and 24d and the gate electrode 22 in prolonged use.